The present invention relates to an oxygen sensor controlling apparatus that controls an oxygen sensor having a characteristic that a sensor output with respect to an air-fuel ratio is rapidly changed between a rich state and a lean state with a theoretical air-fuel ratio being a boundary in response to an oxygen concentration in an exhaust gas of an internal-combustion engine, an oxygen sensor controlling method and a computer readable recording medium.
In the related art, an oxygen sensor that is provided in an exhaust pipe of an internal-combustion engine of a vehicle and detects a rich state and a lean state of an air-fuel ratio in the internal-combustion engine in response to an oxygen concentration in an exhaust gas has been proposed. The oxygen sensor includes a detection element that includes a solid electrolyte such as zirconia as a main body. In such an oxygen sensor, in a case where the oxygen concentration is expressed by an air-fuel ratio of fuel combusted in the internal-combustion engine to air, an output voltage (sensor output) with respect to the air-fuel ratio is rapidly changed in a binary manner with a theoretical air-fuel ratio being a boundary, and thus, by using this phenomenon, it is possible to detect whether the air-fuel ratio of fuel in the internal-combustion engine is on a rich side or a lean side with reference to the theoretical air-fuel ratio. The solid electrolyte that forms the detection element shows excellent oxygen ion conductivity in a high temperature state (activated state) of about 600° C. or higher. Thus, a heater that heats the detection element is provided in the oxygen sensor. Further, by using the phenomenon that element impedance (internal resistance) of the detection element varies according to temperature of the element, a feedback control for energization of the heater is performed so that the element impedance (internal resistance) reaches a target impedance (target resistance) to maintain the detection element at a constant temperature in an activation temperature.
However, in the detection element of such an oxygen sensor, it is known that the element impedance (internal resistance) gradually increases by degradation due to its use or the like. That is, in the detection element after degradation, the element impedance (internal resistance) at the same temperature is relatively high compared with the detection element before degradation. Thus, in a case where the detection element is degraded, if the above-mentioned feedback control of the heater is performed, a control is performed in a direction where the element impedance is decreased, that is, the element temperature is increased to make the element impedance be close to the target impedance. Then, the detection element is excessively heated, and thus, the element temperature rises, which causes a problem that the degradation is further promoted due to the temperature rise.
In order to solve the problem, JP-A-10-26599 discloses an oxygen concentration detection apparatus that includes degradation determination means for determining a degraded state where an element impedance of a detection element is increased and target impedance change means for increasing a target impedance when it is determined that the detection element is in the degraded state. Further, heater-supplied power comparison means for comparing heater-supplied power supplied to a heater and a predetermined determination value is disclosed as a specific example of the degradation determination means.